The desire of the time for the size and enhancing the performance of electronic devices has promoted various rapid progresses in the field of mounting substrate, such as making thinner the wiring of multilayered printed circuit board in which substrate circuits are formed into a multilayered structure, increasing the number of layers and enhancing the density of wiring. As its result, prior multilayered wiring boards in which the wiring layers are connected by through-holes have become incapable of coping with the desire for enhancing the wiring density because of the large area occupied by through-holes. For such a reason, multilayered circuit board of buildup mode is actively studied in the recent years. In a multilayered printed circuit board of buildup mode, the wiring layers are connected one another through minute holes called “via holes”.
As the interlaminar insulating layer thereof, epoxy resin compositions are used because of excellent electrical properties and adhesive properties of the compositions and excellent mechanical properties of the cured products thereof.
In addition, an interlaminar insulating layer is required to be flame-retarded from the viewpoint of safety, as typified by glass fiber-reinforced epoxy printed circuit boards, and they are flame-retarded by the use of halides (an example thereof is the brominated epoxy resin) or antimony compounds. In the recent years, however, the official regulation on the materials using halides such as bromides and the like and antimony compounds is becoming severer. In view of such a state of things, thermosetting interlaminar insulating layers using a triazine ring-containing epoxy resin as a curing agent for epoxy resin have been proposed (JP-A 11-87927, 11-1547, and 11-343398).
A buildup multilayered printed circuit board of photo-via mode using a photosensitive resin as interlaminar insulating layers on which via holes are formed by the photolithographic method has also been proposed. A buildup multilayered printed circuit board of photo-via type can decrease the diameter of via-holes, so that the area occupied by through-holes can be lessened to a great extent and, at the same time, many via-holes can be formed at once. Examples thereof include the negative type photosensitive resin compositions constituted of epoxy acrylate and epoxy resin (JP-A 9-40751, 10-36682, 10-173336); the negative type photosensitive resin compositions which are cured by the use of epoxy resin, novolac type epoxy resin and acrylate (JP-A 11-30855); the negative type photosensitive resin compositions in which an epoxy resin and a resol type phenolic resin are cured with a cationic photo-initiator (JP-A 5-136575); and the negative type photosensitive resin compositions composed mainly of a chalcone-containing bisphenol-epichlorohydrin type epoxy resin (JP-A 8-236945).
The above-mentioned interlaminar insulating layer using a triazine ring-containing phenolic compound as a curing agent for epoxy resin can achieve flame-retardation without use of brominated epoxy resin. However, this type of compositions are thermosetting, so that via-holes can be formed only by heat-curing the composition and thereafter forming the holes one by one by means of carbon dioxide laser. It takes a long period of time to produce a printed circuit board having a number of via-holes by such a method.
Further, prior epoxy resin compositions having photosensitivity have been difficult to flame-retard without use of brominated epoxy resin. Further, the above-mentioned photosensitive resin compositions are of negative type in which an area exposed to active energy beam cures and the unexposed area is removed by the process of development. Accordingly, the active energy beam is absorbed into resin and diffused at the time of exposure and the extent of cure differs with depth of interlaminar layer, due to which the via holes formed by development assume an inversely tapered shape. This can deteriorate the throwing power in the subsequent copper-plating step and thereby cause a defective connection.